The disclosure relates generally to improvements in enhanced oil recovery processes in the field.
The amount of crude oil that can be extracted from an oil field can be increased by the use of various enhanced oil recovery (abbreviated EOR) techniques. Condensed carbon dioxide (CO2) has been used for 40 years as a profitable EOR technique even though CO2 flooding does not recover all of the oil in the formation regardless of whether the reservoir has been previously water flooded. One of the causes for this low oil recovery can be traced to the low density and viscosity of condensed CO2. The viscosity of dense liquid or supercritical carbon dioxide (also referred to herein as condensed carbon dioxide) at typical CO2 flooding conditions is ˜0.05-0.10 cP and is much lower than that of the oil and brine in the formation (0.5-10 cP) which results in an unfavorable mobility ratio (M>1). This leads to viscous fingering, early CO2 breakthrough, high CO2 utilization ratios, elevated CO2 production and re-compression, and depressed oil production rates even though the CO2 flood is conducted at or above the minimum miscibility pressure (MMP). These problems can be exacerbated when the injection is completed in two or more producing zones. The low viscosity of condensed CO2 promotes its flow into the more permeable layers that have been effectively water flooded, while disappointingly small amounts of condensed CO2 enter the low permeability zones that contain more recoverable oil. Finally, the low viscosity of condensed CO2 in reservoirs with communicating layers exacerbates gravity override. All these elements lead to low oil recovery and less than optimum storage capacity.
A CO2 thickener would reduce the mobility of condensed CO2, which would lead to maximizing the ratio of oil produced to CO2 injected and increasing the capacity of the formation to retain CO2 at the end of the project. However, thickening CO2 to suppress early breakthrough of CO2 in EOR is not easy because condensed CO2 is a poor solvent for most organic molecules. Attempts to increase the viscosity of condensed CO2 and raise it to a level comparable to that of oil via dissolution of high molecular weight polymers, associating polymers, or small associating molecules have been summarized in reviews and papers. R M Enick “A literature review of attempts to increase the viscosity of dense carbon dioxide,” Oct. 10, 1998; “A Laboratory Investigation of Viscosified CO2 Process, SPE Advanced Technology Series, SPE-20467-PA, 1993 1(1), and D K Dandge, J P Heller “Polymers for Mobility Control in CO2 Floods, SPE International Symposium on Oilfield Chemistry, SPE-16271-MS, 1987.
In such previously attempted methods, typically, CO2 is purified to >95% purity and then subsequently thickeners and/or cosolvents are added to the purified CO2 so that it can be recycled in EOR. However, the purification and subsequent addition of thickeners and/or cosolvents are resource-intensive and expensive steps. There is a need in the field for cheaper and efficient methods of re-utilizing CO2 for enhanced oil recovery.